credentials: Support ALTSPerRPCCreds in DefaultCredentialsOptions

[rockspore]

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 70.58824% with 5 lines in your changes missing coverage. Please review.

Project coverage is 81.20%. Comparing base (f8e5d8f) to head (c5db6fb).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
credentials/google/google.go	70.58%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7775      +/-   ##
==========================================
- Coverage   81.32%   81.20%   -0.13%     
==========================================
  Files         368      368              
  Lines       36779    36799      +20     
==========================================
- Hits        29912    29883      -29     
- Misses       5635     5666      +31     
- Partials     1232     1250      +18     

Files with missing lines	Coverage Δ
credentials/google/google.go	77.02% <70.58%> (+1.58%)	⬆️

... and 23 files with indirect coverage changes

[rockspore]

@dfawley This is for the ALTS token design. The Java equivalent is grpc/grpc-java#11634. Could you help review this PR?

[dfawley]

Name should match

[rockspore]

Thanks for the catch. Fixed.